1. Technical Field
The invention relates to audio amplifiers, and particularly to distortion compensation in amplifiers for vehicle audio systems.
2. Related Art
Automotive audio systems include one or more amplifiers that provide gain for audio signals. The gain of an amplifier depends, at least in part, on the amount of power supplied to the amplifier. Because the supply voltage varies over a range of several volts, the amount of power that is available to the amplifier will also vary over a wide range. If the voltage in the vehicle is insufficient for the amplifier to accurately amplify the input signal, the amplifier may clip, and the audio signal from the amplifier will be distorted. Accordingly, amplifiers may be designed to control audio output according to fluctuations, or variations, in the limits of the battery supply voltage, or minimum expected available power.
To preclude the amplifier from being overdriven, an amplifier may control the audio output based on analog or digital signal processing techniques. For example, an amplifier may monitor the audio signal to modify portions of the audio output signal that may exceed a predetermined threshold. Analog amplifiers may use an analog clip detection circuit, which utilizes a voltage controlled amplifier to reduce overall gain during portions of the audio output signal that exceeds the vehicle voltage. Since clip detection of monolithic power amplifier integrated circuits is typically shared over two or more channels, the analog control maintains a relatively constant amount of distortion, but may not control the distortion for each channel independently.
A digital amplifier may use a digital signal processor (DSP) to predict an output signal level on the channels of a digitally-sampled audio signal. The DSP reduces the gain for the portions of the signal that exceed a programmed threshold. A DSP may independently control each channel to tailor the amount of distortion. However, the threshold may be set, or hard-coded, into the digital signal processor. To account for variations in power supplied to the amplifier, the threshold may be set much lower than the actual level that may be available from the battery. Accordingly, hard-coded distortion limiting using a DSP may not provide an optimal amplification of the audio signal for a wide range of battery supply voltages.
Therefore, there is a need for a distortion compensation for automotive amplifier applications.